High-temperature cleaning of ferrous metal



Jan. 13, 1953 c. CONE ET AL HIGH TEMPERATURE CLEANING OF FERROUS METAL Filed June 4, 1948 .2 SHEET$SHEET l CARROLL CONE JACK HUEBLER C. CONE ET AL HIGH TEMPERATURE CLEANING OF FERROUS METAL Jan. 13, 1953 2 SHEETS-SHEET 2 Filed June 4, 1948 H /I \r wk I H I m ,3 H 4/ mm Mm G om R 3 G m S i 5% VI 9% mm mm 3 9? IE .S & mv mm N. E S 8 IMVENTORS CAR ROLL CONE ATTORNEY Patented Jan. 13, 1953 UNITED STATES 2,625,495 IgQH-TEMPERATURE CLEANING or nanovs METAL Holman-Ohio, a corporation' ofgQhio en me 4, rsael i 1. w 7 (01. 134-2) The invention relates to a method of cleaning the surface of aferrous'rnetal hyfre novingcan bonaceous matter. and iron oxides in preparation for a coating process jsuchastinning, galvanizing, plating or painting, and particularly to the cleaning of cold reducedf s ixeelfst'rip.

In the manufacture of cold reduced steel strip, the ingot is first .redu cedpvlhgt rollingit'o a'gauge about four to ten' times that'iofjthejfinaljgauge. The resulting .strip i s,pi ckled tov remove oxide scale formed during l ie hot rollingoperation, washed .with water to remove picklin residues and then oiled in preparation for coldrolling.

After cold rolling, the steel strip carrieslonits surface a residue of oil andotherorganic matter which must be removed before thefstrip, islready for coating. In order .jtolremove fsuch organic matter, it has been customary'heretofore toi'wash the strip in a solvent ,or, alkaline detergentlsolution, with or .withoutelect'rolysis.' Howevenjsince the surface of the. strip f'entering'thecoldrolling mill is rough and-deeply etched, impuritiessuch as oxides, pickling iresid is-sand organioilmatter become imbedded inthe.surfacf ofithe' iirietal during cold rolling. ":.Was1iifig}ltliej'surfacei'oflthe cold reduced strip, .ev'ii ;,w 1eii aceampamed f'by electrolysis, removes!onlysuperficial mpiirities and does not remove embeddedirnpurities. ,If the strip after 'washing-is 'introduced int ci v at of a molten coatingflmetalas in a galvanizing operation, the rise in temperature causes the embedded oxides and organ c-niatter to generate gases, such, as carbon dioxi de an d. water vapor, which are trapped in the.rnetalcoating causing porosity and poor adherence. The embedded i n purities also interfere with the wetting and allo yn of the co in met lw t the-beseme a chemically a e. c atin smeta suq s s 2 9. alum um also mayurea twithth embedd ilr fiz id s t rm oxides. f. th coat n met Whk interfere with the. adherence; of the coating.

The principal, object of, 'the jnventionis to provide an-imprcoved method pf cleanin -,ffi flious metal.

More specific objectsvuand advanta es ar apparent from the description, in; which, reference is had to the aocompanyingdrawings illustrating various forms of apparatus for usein thepractice of the invention.

Figure I of qr in s e. a d a ramm ti representation ,of, anapparatus for processing cold reduced steel stripzin 'ac rdance-vviththe present invention Figure II is a similar. viewfishowing a preferred form of apparatus.

m2 Figure III is a horizontal section talren on line III'III of'FigureII.

Figure IV is a detail showing a modificationof I the apparatus illustrated in lf'igure II.

'These's'pecific dratvingsiand the specific description that follows" disclose and illustrate, but are not intendedtolim-it the scope of the invenf t United States Patent No. 2,238,930, issued to John J. Turin, suggests that'a 'stripofa ferrous metal be passed through 'apreheating furnacein which'it isjexposedto an' openfiame, to burn off oil, and then-passed throughamuffle furnace in which it is exposed to anatmosphere formed by burning a hydrocarbon fuel withchlorine and air, to convert iron -oxides "to iron chlorides. If the step of exposingthe stripjto' an openjfiame to burn off organic n er had been omittedjfro'm the process disclosedim thisi patent, the" only now w y 1,.ca ;r in e.o1 flth o would have-been torerno the organici matter from the strip hy the usf ,peliininary washing u 1.1,; ,,s. t at,

' It has npt b eenknovvn ,to be possible heretofore to ren iove car aceous matteriand" iron oxides simultaneouslyfrom the urf'acei'ofv a ferrous metal in a single operatibnjTh present invention is.basedjupon ihediscovery that it is I possibleto Vremoveicarbofnaceous'matter and iron oxides simultaneously, rmmnre surface of afferrous rnetalfby heating therfiietal and exposing it to atmosphe're thatfi's oxidizing tofcarb'o'n and d'eoxi'dizingto ir 'n tjoxids. s used herein the term deoxidizin'gl iro "Xides'" is' 'i'ntended'to in clude conversion .offiron oxides to 'ehior'mes."

The ferrousme'talto 'be'elane'd in the practice of the; present invention" preferably is preheated before it is introduced intoIthe' cleaning atrnosph'ere. The .heatlfigf of the metal can be carried out more economically py usin'g a preheater than by intro'ducinglfthe cold metafd i re'ctly into the hot cleaning atmosphere.

Moreover, T h'reasiiqnsmax ere e se y ic e n'jimberawre hasemeiatm 'n r that deoxidizesironoxides at the elevated operating te pe u m r-a tual vQX iZB nii at 9??? t mper ture il snt einiro uqti r q -i meia s irectliaintou hefic ea in atmosphere rnay s lovv glpyvn the process by, causdrop in ternperature, it isdesirablealso toT'c'ool the metal outside "ofthe cleaning atmosphere.

' ing initial formation'of iron xides which must be q i'efii gfih m i lha x ac d; t operating temperature} Ifthe conditionsare such that the reactions are liable to bereversed bva astnos The preheating or outside cooling should be done in a non-oxidizing medium in order to avoid the formation of iron oxides. One convenient method of preheating consists in passing a continuous strip of the metal into the cleaning atmosphere through a bath of molten lead or of a molten salt. Cooling also can be carried out by passing a strip of the metal out of the cleaning atmosphere through such a bath, but when so carried out may produce changes in the metal of the type which ordinarily result from quenching, so that other methods which provide more gradual cooling are ordinarily preferred.

The preheating or cooling may be. carried out also by passing a strip of the metal through a preheating chamber, for example a mufile furnace, or a cooling chamber, containing a nonoxidizing atmosphere. Although'nitrogen may be used as the non-oxidizing atmosphere in the preheating or cooling chamber, it is usually preferable to use an atmosphere consisting of about 90 mol per cent of nitrogen and mol per cent of hydrogen.

During such preheating, oil and other organic matter on the surface of the metal may be carbonized by decomposition into carbon and free hydrogen or water vapor. In any case, the organic or carbonaceous matter is oxidized to gaseous oxidation products upon exposure to the cleaning atmosphere.

The temperature of the cleaning atmosphere (hereinafter referred to as the operating temperature) used in the practice of the present invention may range from dark red heat (6'70- 850 F.) upward to well above 1300 F., for example, to about 1700 F., depending upon the particular metallurgical heat treatment that may be desired to fit the ferrous metal for a specific use. The preferred operating temperature is in the range from about 1000 F. to about 1500 R, which is the range of temperature to which cold reduced steel strip ordinarily is heated in annealing. When an operating temperature in the latter range is employed in cleaning cold reduced steel strip in the practice of the invention the desired annealing is accomplished simultaneously with cleaning.

In one embodiment of the present method, the cleaning atmosphere deoxidizes the iron oxides on the surface of the metal by converting them to chlorides. In order to accomplish that result, the cleaning atmosphere may contain a chlorideforming gas such as chlorine. The preferred chloride-forming gas is hydrogen chloride. The atmosphere introduced into the cleaning chamber may be an atmosphere containing gases that react to produce a chloride-forming gas. For example, the atmosphere introduced may contain phosgene and water vapor, which react to form hydrogen chloride and carbon dioxide.

One atmosphere that may be introduced into the cleaning chamber in the practice of the invention contains from to 45 mol per cent of hydrogen chloride, from 10 to 12 mol per cent of carbon dioxide, not more than 2 mol per cent of oxygen and not more than 3 mol per cent of water vapor, the remainder of the atmosphere consisting of nitrogen. If desired, the composition of this atmosphere may be rendered less oxidizing by omitting the free oxygen and adding not more than 3 mol per cent of carbon monoxide and not more than 2 mol per cent of hydrogen,

with or without a slight reduction in the carbon dioxide content.

When the cleaning atmosphere contains a chloride-forming gas the metal preferably is kept at a temperature not less than about 1300" F., to prevent the solidification of ferrous chloride and thus facilitate its removal from the surface of the metal. In the cleaning of cold reduced steel strip in accordance with the present invention, there is no substantial oxidation of the iron and the amount of ferrous chloride produced by reaction with a chloride-forming gas in the cleaning atmosphere is relatively slight as compared with the amount of ferrous chloride that would be produced if it were necessary to remove a heavy coating of iron oxide from the metal. Thus in the present method any coating of ferrous chloride left on the surface of the metal is so slight that it sufiices to coat the metal with a chloride flux before introducing it into a galvanizing pot. The necessity for coating the metal with a chloride flux may be avoided by passing the metal through a lead bath (without exposure of the metal to air) at a temperature not less than 1300 F. to remove the slight coating of ferrous chloride. In contast to the slight amount of ferrous chloride produced in the practice of the present method, the process described in the Turin patent involves heavy oxidation of the strip by the action of the open flame used to burn off the organic matter, and the removal of the resultant heavy coating of iron oxides produces a relatively large amount of ferrous chloride. Thus the strip leaving the cleaning atmosphere in the process of the Turin patent has a relatively heavy coating of ferrous chloride which can be removed satisfactorily only by a washing operation.

In a preferred embodiment of the present method in which the formation of a coating of ferrous chloride on the surface of the metal is avoided altogether, a chloride-forming gas is present in the atmosphere introduced into the cleaning chamber in a concentration low enough to form a final concentration of ferrous chloride vapor less than saturation at the operating temperature, i. e., the concentration of chlorideforming gas in the incoming atmosphere is low enough so that after equilibrium at the operating temperature has been reached in the conversion of the chlorine content of the chloride forming ga to ferrous chloride, the atmosphere still is not saturated with ferrous chloride vapor, and all of the ferrous chloride formed volatilizes. Then, in order to prevent the ferrous chloride from recondensing on the surface of the metal, the metal may be removed from the chloride-containing atmosphere to a cooling chamber as hereinbefore described.

In the apparatus illustrated in Figure I, cold reduced steel strip 2 is unwound from a coil on a pay-out reel 3. At this point, the strip, as obtained from the reduction rolls, has a coating of .rolling oil and is hard from cold working. A non-oxidizing preheating medium is provided in the form of molten lead contained in a tank 5, which is heated by a furnace 4 in which the tank is positioned. The strip passes downward from an overhead roller 6 to a submerged roller 1 in the lead bath and thence horizontally to a submerged roller l0. As the strip enters the lead bath, any loose dirt adhering to the oil on the strip is wiped off and floats on the surface of the lead. Some of the oil also is wiped off, and the remainder of the oil is carbonized on the strip. The lead bath also heats the strip to the desired annealing temperature and preheats the strip for the cleaning operation.

aria-455 F m i em ler I". e. tr ba "u w rd wi ms re o. a rimio anthe re 1; having a lower extension l2 which "dips into the molten lead to, form afgas-tight seal. The retort, which contains the cleaning ;atmosphere, is surrounded by a furnace lfl forfmaintainihgthe retort at the desired operating temperature By the time when the"s trip-reaches an overhead roller M at the top of the retort, the cleaning operation has been completed and the strip is ready for cooling. An atmosphere of the iCOIllposition hereinbefore described, consisting principally of hydrogen chloridefcarbon dioxide, and nitrogen, is admitted at the top of the -retort through a supply =pipe l'B'and fio ws d'giwnward through the retort inc'ohtact withthe upcoming strip. The cleaning atmosphereleaves"thelow'er end of the retort through 'awastegas pi egss. 7

From the top of the "retort, the'stripfpasses into a'closed cooling chamber comprising-a relatively hot initial portion I5 ahda relatively'cool final portion H. The cooling chamber co'ntains rollers 28 to 24 for guiding the strip in its passage through the chamber. In" order "to prevent oxidation of the strip in 'the'cooling chamber a suitable protective atmosphere is mainteined-inthe chamber. In the apparatus illustrated in Figure I such a protective 'atinosp'here'is maintained by permitting some of the atmosphere from the supply pipe IS to flow into the cooling chamber and out through a vent '32'irrthe coolport'ion I! of the chamber.

Strip cleaned in this apparatus, before it has cooled below galvanizing temperature, may be passed directly from the cooling chamber into a galvanizing pct without-any intermediate treatment other thanpass'age through a layer of molten flux contained me flux box 19. Rollers 25 to 28 conduct the strip through andfrom the galvanizing pot. From the pot the strip passes through a cooling zone 29 to an overhead roller 30 and thence toa'wind-up reel-3 l. I

When the strip to be cleaned is passed through an elongated cleaning chamber and the cleaning atmosphere flows countercurrent to the travel of the strip, the composition and temperature of the atmosphere at the point where the atmosphere leaves and the strip enters the chamber is of no consequence provided that the strip subsequently remains fora sufiicient length of time in a portion of the cleaning chamber in which the atmosphere is of a composition and temperature herein'specified. :Prfeferably, however, the cleaning atmosphere throughout the entire cleaning chamber hasits temperature-and com position within the ranges "specified herein so that the speed with which the strip travels through the cleaning chamber may be the maximum speed that allows time for oxidation of the carbonaceous matter and deoxidation of the iron oxides. h U U i Figure II illustrates apparatus in which cold reduced steelstrip 34is unwound from a coil on a pay-out reel 35. A preheating zone is provided in the form of an upright tunneltl, which is heated by a furnace 33; in which 'the tunnel is positioned, and which contains a non-oxidizing atmosphere. The strip passes beneath a roller 36 and upward over'an overhead'roller 48. As the'strip enters the manner through a slot 37 it is heated was elevated temperature'in anatmosphere that is non-oxidizing and 'may be slightlyreducing; nitrogen may be used as the atmosphere, but' a manner '90' mol'per'cent nitrogen and 10 mm percent hydrogenis' usually preferred, "Such fah' at l U the top of thetunnel S h I V and fi wsd w wardfih q l h hawed tact t in? i iflg And-s her leaves the lower end of the tunnel throughawaste gas p e, t From he ,t fi ignn l f t pli' sfi'S through a slot 42, over "a'rollerj'4 ljand downward into upright retort 49 "having a lower-extension 59 which dips into a moltenfllea bathcontai ed in a tank 5|, which'is heated y afurnace i'lfi in which the tankis-po'sitioned. The retort fl, which contains the cleaning atmosphere, is maintained at the desired operatingtemperature by the furnace 38. When thfistrip reaches the lead bath belowthe retort,'thefg1aseoiis clea' tion has fb'een, completed and theistr p s clean except for a coating "of "fr'reu' "chlo de w an is removed in the lead Ib'ath. Tlife V ,7 u mosphere isfadmitted'at thebott'orn'o the through '"a supply pipe 48 an flows up I through the retort in contact with the'dowriward moving strip. The atmosphere leaves the upper end of the retort'through"ajjwaste gas pipe 41. The strip is conductedthr'oughthe lead bathby rollers 53 and 54 The ferrous chloride in the lower extension of the retort 49' isliept from attaining an'unduly high level byano'verflo 'w l5.

From the lead bath, the strip 'passsu itofa closed cooling chamber 'z o'r'nprising a mien ely hot initial portion 55 a relatively cool portion '55. The intermediate portion '01? "the cooling chamber contains rollers 51 to GU-Q for guiding the strip in its passage through the chamber. In order toprevent oxidation of the strip in the cooling chamber asuitable protective atmosphere is maintained in the chamber, -In the apparatus illustrated in ZEigureqII- such a protective atmosphere is admitted at the cool end of the cooling chamber-through-a supplypipe 62 and flows through the cooling chamber and out through a waste gas pipe 6].

Strip cleaned in this apparatus; beforeiti-has cooled below galvanizing temperature, may be passed directly from the 'cooling chamber into a galvanizing pot 68 without any intermediate treatment. Rollers 63 to 61 conduct thestrip to, through and from the "galvanizing pot: From the pot the strip. passes to an overhead "roller "69 and thence to a wind-'upre'el 70.

Figure-III shows a section through the furnace 38. The strip 34 isre'presentedin'sectiorr inside the tunnel 39 and retort 49.

Figure IV illustrates an alternative connection between the cleaning zone 49 and the cooling zone 55 which connectionfeliminates the lead bath contained in the tank"'5l. This alternative connection can be used when the atmosphere in, the cleaning'zon'e is 'such 'tliat'carbonaceous matter is oxidizedand iron oxides are deoxidi z'e'd simultaneously Without the formation of a eoating of ferrous chloride, as f-de'scribed herein. When this alternative "connection =is used, -the strip 34 passes downward from the retrt 4'9, through a slot H, beneath rollers"lzand'liijand upward into the cooling'zo'ne55. 'In the modification illustrated in Figure IV the protective atmosphere of the "cooling z'one isf exhausted through a waste gas pipe '14. When the cleaning atmosphere that is used "isbril diz'ing o' -carb'onthis is true because the com aceous matter because *"ofthe' pre'sencef oi ing atmosphere hasbe'enregulated sd tlilttiertain equilibrium conditions are exceeded (causing a tendency for the oxidation of carbon) at the same time that certain other equilibrium conditions are exceeded (causing a tendency for the reduction of iron oxides). When it is desired to oxidize carbon (at atmospheric pressure) by the action of water vapor, the initial concentration of H20 should exceed (0.01) (as used herein, a gas concentration expressed as a figure in parentheses refers to the partial pressure of that gas in atmosphere), and the concentration should at all times be such that coma 12,750 (1) (H2O) t+460 in which (CO), (H2) and (H20) are the gas concentrations of carbon monoxide, hydrogen and water vapor, respectively, and t is the operating temperature in degrees F. When it is desired to oxidize carbon by the action of carbon dioxide, the initial concentration of CO2 should exceed (0.05), and the concentration should at all times be such that in which (CO2) and (C) are the gas concentrations of carbon dioxide and carbon monoxide. respectively, and t is the operating temperature in degrees F. When it is desired to deoxidize iron oxides by the action of hydrogen, the initial concentration of H2 should exceed (0.05), and the concentration should at all times be such in which (H2) and (H20) are the gas concentrations of hydrogen and water vapor, respectively, and t is the operating temperature in degrees F. When it is desired to deoxidize iron oxides by the action of carbon monoxide, the initial concentration of CO should exceed (0.05) and the concentration should at all times be such that 00 1800 (4) col t+460 in which (CO) and (CO2) are the gas concentrations of carbon monoxide and carbon dioxide, respectively, and t is the operating temperature in degrees F. When it is desired to deoxidize iron oxides by the action of hydrogen chloride, the initial concentration of HCl should exceed (0.01), and the concentration should at all times be such that (H2) 9770 (a) (HCl) i+460 in which (H2) and (H01) are the gas concentrations of hydrogen and hydrogen chloride, respectively, and t is the operating temperature in degrees F.

As explained hereinbefore, it is sometimes possible to clean steel in the practice of the invention under such conditions that the cleaning atmosphere exhausted from the cleaning zone is not within the limits defined by Inequalities 1 to 5. When this is true, the equipment is being operated inefiiciently, and more strip could be cleaned by keeping the entire cleaning atmosphere within the prescribed, limits and moving the strip through the cleaning zone at a greater speed. Consequently, in order that the equipment may be operated eificiently, it is preferred that the gas exhausted from the cleaning zone be within the prescribed log 7.543

logic 4.44:

In the analysis of gas compositions that are used in the practice of the invention it is necessary that certain precautions be taken to ensure accurate results. It is understood that a mixture of C0, C02, H2, H20 and HCl is capable of any of a number of equilibria. What the equilibrium is for any mixture that contains these materials is a function of temperature, so that a gas composition that is not within the limits prescribed for use in the practice of the invention could, if analyzed at a difierent temperature under equilibrium conditions, appear to be one within the limits. Conversely, a gas composition within the prescribed limits could, if analyzed at a different temperature under equilibrium conditions, appear not to be within the limits. Thus it is important that the gas analysis be made in such a way as to avoid these possible sources of error. It has been found that changes in the composition of the mixture of gases used as the cleaning atmosphere occur at an extremely slow rate at room temperature. Therefore, the expedient of rapidly cooling a sample of the atmosphere ensures suificiently accurate results; rapid cooling to room temperature has the eifect of freezing the gas composition. Any HCl in the atmosphere should be removed before the rapid cooling to prevent it from dissolving in condensed water vapor to form a hydrochloric acid solution which then absorbs CO2 so as to make analysis more difiicult.

The atmosphere that is maintained in the cooling zone should not be oxidizing to the cleaned steel surface, and best results are frequently obtained when this atmosphere is slightly reducing. Thus, it is possible to conduct the cleaning operation with a nitrogen atmosphere in the cooling zone, but it is usually preferable that the atmosphere be slightly reducing, a mixture or" mol per cent nitrogen and 10 mol per cent hydrogen being a desirable reducing atmosphere.

Because the cleaning atmosphere that is used in the practice of the invention may be eiiective at a given operating temperature, but either oxidizing to steel or reducing to carbon at a slightly lower temperature, it is usually desirable to avoid any leakage of the cleaning atmosphere into either the preheating zone or the cooling zone. This can be accomplished readily by operating the cooling and preheating zones at a pressure slightly greater than the pressure of the cleaning zone, so that any flow of gas from one zone to another will be toward the cleaning zone. Except for the slightly higher pressures in the cooling and preheating zones, it is usually desirable to conduct the cleaning operation at or about atmospheric pressure.

Ferrous metal can be cleaned, in the practice of the invention, without pretreatment, carbonaceous matter and iron oxides being removed chemically as described herein. However, particularly when the metal to be cleaned has a heavy covering of carbonaceous matter which is loosely adherent to the surface of the metal, it may be advantageous to subject the metal to a pretreatment in the form of a washing operation to remove surface carbonaceous matter so that there is less to be removed by the method of the invention, which is particularly efiicient in that it removes carbonaceous matter that the washing operation does not reach, and simultaneously reduces iron oxides.

In general, the minimum time that the ferrous metal should remain in contact with the cleaning atmosphere depends upon the operating acaacoa temperature and upon the amount oi carbona ceous material and iron"'oxides "to be removed from the metal. Thus, using. an operating temperature of about 1000 F. it is practical to conduct. ferrous metal" that "has been washed to remove some surface carbonaceous matter through the cleaning. atmosphere at such a rate that it is subjected to cleaning conditions for a time as short as about 30 seconds, although it is usually preferable to allow about an extra five seconds to make certain that the metal will be thoroughly cleaned. When the "operating tem perature is about 15008 F. such metal can be conducted through the cleaning atmosphere at such a rate that it is subjected to cleaning conditions for a time as short as about five seconds, although it is usually preferable to allow about an extra two seconds to make certain that the metal will be thoroughly cleaned. When the metal to be cleaned has a heavy surface coating of carbonaceous matter it may be desirable to maintain the metal in' contact with the cleaningv atmosphere for about twice the time indicated'for'inetal that has been washed. When intermediate operating temperatures are used, the minimum time. that the metal should be"iri"dontact ivvith'the" cleaning atmosphere is intermediate between the times indicated. There is. no theoretical reason why the metal cannot be left in contact with the cleaning atmosphere for. comparatively long periods of time, provided that the composition of the atmosphere is within the limits described. HOW? ever, in practice it is usually desirable to operate the cleaning equipment at such a rate that the maximum amount of metal is. cleaned during. the operation. Therefore, it is ordinarily preferable a the me al be conducted through the cleaning atmosphere, at about the maximum rate that es emcient cleaning.

A p efe red embodiment of the invention in.- es e use o a. clea in atmosphere which mp ises ydro en and water, vapor. An exam? p o such. an atmos h re is one. containing 20v mol per ent of d o en 5 mcl per c nt of water po a 7 5 mo e cent o t o en Such an m s e e can he use within the preferred ran e of o e t n empe a r s e, at tern: peratures between about 1000 F. and about 1500 F.

The way in which Inequalities 1 through 5 can e us d o: d te m ne r t n c; dawn wh c will be simultaneouslybx izinjg'tj rbonaeebus matter and reducing to'iron oxides is'illu'strat'e by the following calculations:

Cons de an operat n temperature of 1201)".

and ah a mc h re th that monoxide, carbon'dioxide, andahiluent, such as nitrogen.

F om n quali y a (G9) 19cc r m ine alit 2 2663 6 From the ratios,

and

components depend upon temperature.

(CO2), igoooimf Solution oi these two equations gives the limits of concentration within which, at1200? R, carbon dioxide can be present in] a gas composition in whic'h'the partial pressure of carbon monoxide is 0.07 atmosphere, that is's'imu'lta'neou'sly oxidizing to carbonaceous matterfand deoxidiz in'g to iron oxides, if in addition to. the carbon monoxide and carbon dioxide the composition contains only an inert gas such. as nitro en. By similar calculations, assuming 'difieren't temperatures and different carbon monoxide concentrations. the limits oi other suitable gas compositions. that consist of carbon monoxide, carbon dioxide and an inert'gas' can be calculated.

If the gas composition that is selected for use in the cleaning zone is one whose components are capable of reaction, 1. e., are out of equilibrium, the equilibrium composition of the gas at the desired operating temperature should be deterined in order to ascertain. whether or not the equilibrium composition is within the limited defined by Inequalities 1' through 5. so that'the gas, at equilibrium, is simultaneously oxidizing to carbonaceous matter and deoxidi'zing to'iron oxides. For example, if the gaseous atmosphere contains H2, H2O, CO and CO2, the stable mixture is one represented by the dynamic equilibrium.

The equilibrium concentrations of the various It is usually preferable that'the cleaning atmosphere be one which, when it reaches equilibrium at the operating temperature, is simultaneously'oxidizing to carbonaceous matter and deoxidizing to iron oxides so that there is not the possibility. that, during the cleaning reaction, the gas "will, in approaching equilibrium; become, for example; oxidizingtoiron.

A trial and error method may be used for detel-mining the gas compositions that are practical for use in the practice of the intention; instead of the method of calculation indicated in'the pr ceding paragraphs. This is done by supplying'to a cleaning zone a gas of a composition that is known, fr onsideration of the relationships shown in Inequalities 1 through 5, tob'e simu'l taneously oxidizing to carbonaceous matter. and x dizins to iron oxides, pass n the as through the cleaning zone at the desired operats tem erat in conta t w th the; metal 'tobe cleanedand the iaiiai outcomi'ns-"gas. it is non determined ..i not the-enemas indicates that the gas (according to Inequalities 11 1 through is simultaneously oxidizing to carbonaceous matter and deoxidizing to iron oxides. This method is usually found to be simpler than theoretical calculations based upon equilibrium considerations and rates of reaction.

As has been stated hereinbefore, a gaseous composition that is oxidizing to carbon and deoxidizing to iron oxides at a given temperature may be oxidizing to both carbon and iron oxides at lower temperatures. For example, the relationships in Inequalities 2 and 4 are such that, when t is less than about 900 F., there are no compositions of carbon monoxide, carbon dioxide and an inert gas that are simultaneously oxidizing to carbonaceous matter and deoxidizing to iron oxides, because the minimum concentration of CO2 that is possible (to give a gas that is oxidizing to carbonaceous material) is higher than the maximum concentration of CO2 that is permissible (to give a gas that is not oxidizing to iron). As a consequence, it is usually preferred that the metal to be cleaned be heated, then exposed to the cleaning atmosphere, at a temperature in the operating range, and then cooled in an atmosphere different from the cleaning atmosphere. The cooling atmosphere should be non-oxidizing to the metal, and preferably slightly reducing.

A preferred embodiment of the invention (to which the equipment generally illustrated in Figure II is well adapted) involves the use of a cleaning atmosphere that contains 1101 in such an amount that it converts iron oxides to ferrous chloride, but does not produce an amount of ferrous chloride sufiicient to saturate the cleaning atmosphere with ferrous chloride vapor at the operating temperature. In practicing this embodiment of the invention, the modification illustrated in Figure IV is used, because the ferrous chloride formed by reaction of the 1101 with iron oxides is vaporized and carried from the equipment with the cleaning atmosphere, so that little or no ferrous chloride remains on the cleaned metal and use of a lead bath is unnecessary.

The instant invention is of particular value because it provides a method for cleaning a ferrous metal that is at the same time efficient, capable of application to simple equipment, and inexpensive (as compared with know ways of cleaning ferrous metals).

Molten metal or salt baths can be used in place A of inert gas atmospheres as either preheating or cooling media.

Example 1 A gas composition comprising carbon monoxide, carbon dioxide, hydrogen, water vapor and nitrogen can be prepared by burning methane in air according to the following procedure:

Methane is burned in air (using a 1:6 volume ratio of methane to air), and the products of combustion are partially dehydrated by cooling them to 45 F. The resulting gas composition, which contains (per 100 mols of gas) 10.2 mols of CO, 5.2 mols of CO2, 12.5 mols of Hz, 1.0 mol of H20 and 71.1 mols of N2, is reacted to equilibrium at 1250 F. (about 30 minutes). At equilibrium the gas contains (per 100 mols of gas) 11.4 mols of CO, 4.0 mols of 002, 11.3 mols of Hz, 2.2 mols of H20 and 71.1 mols of N2. This mixture (the equilibrium mixture of 1250 F.) is simultaneously oxidizing to carbonaceous matter and deoxidizing to iron oxides, and can be used to clean ferrous metals by simultaneously oxidizing carbonaceous matter and deoxidizing iron oxides, care being taken to use a sufiicient quantity of the g S0 12 that all the carbonaceous matter and iron oxides are removed from the metal.

Example 2 A ferrous metal is cleaned by an atmosphere that is simultaneously oxidizing to carbonaceous matter and reducing to iron oxides, prepared as follows:

The cleaning atmosphere is prepared by burn ing a gaseous mixture of methane (1 part by volume), chlorine (2 parts by volume) and air (3.8 parts by volume), and combining 15 parts by volume of this gas with parts by volume of another gas prepared by burning methane (1 part by volume) with air (6.67 parts by volume) and partially dehydrating the combustion products by cooling them to 40 F.; the resulting cleaning atmosphere contains (per mols of the atmosphere) 6.3 mols of CO, 7.5 mols of CO2, 8.3 mols of Hz, 0.7 mol of H20, 70.3 mols of N2 and 6.9 mols of HCl.

If this cleaning atmosphere is reacted with iron oxides at 1250" F. until equilibrium is reached, the final composition of the gas would consist of (per 100 mols of original gas) 8.3 mols of CO, 5.5 mols of CO2, 7.0 mols of Hz, 2.8 mols of H20, 70.3 mols of N2, 5.3 mols of HCl and 0.8 mol of FeClz. By means of Inequalities 1 through 5 it can be shown that this final composition is oxidizing to carbonaceous matter but not oxidizing to iron. The concentration of FeClz in this final composition is not sufiicient to saturate the atmosphere with that component at the operating temperature. Therefore, this cleaning atmosphere can be used at an operating temperature of 1250 F., and, even if the reaction with iron oxides goes to equilibrium, the final gas is still not oxidizing to iron and is still not saturated with ferrous chloride. results in a gas within the indicated ranges of composition. In the use of this cleaning atmosphere in the apparatus illustrated in Figure I or Figure II, the rate of flow of the cleaning atmosphere should be just great enough to remove all the iron oxide from the metal at 1250" F. Such a rate of flow is ordinarily enough also to remove all carbonaceous matter from the metal.

Example 3 A ferrous metal is cleaned by an atmosphere that is simultaneously oxidizing to carbonaceous matter and reducing to iron oxides, prepared as follows:

The cleaning atmosphere is prepared by burning a gaseous mixture of anhydrous ammonia and chlorine and diluting with partially dehydrated products of the complete combustion of ammonia with air to produce a gaseous mixture containing (per 100 mols of the cleaning atmosphere) 0.3 mol of H20, 95.7 mols of N2 and 4.0 mols of HCl.

If this cleaning atmosphere is reacted with iron oxides at 1250" F. until equilibrium isreached, the final composition of the gas would consist of (per 100 mols of original gas) 0.9 mol of Hz, 0.3 mol of H20, 95.7 mols of N2, 2.2 mols of HCl and 0.9 mol of FeClz. The concentration of FeCl: in this final composition is not sufficient to saturate the atmosphere with that component at'the operating temperature. Therefore, this cleaning atmosphere can be used at an operating temperature of 1250 F., and, even if the reaction with iron oxides goes to equilibrium, the final gas is with ferrous chloride.

Of course, less reaction with iron oxides Example 4 A ferrous metal is cleaned by an atmosphere that is simultaneously oxidizing to carbonaceous matter and reducing to iron oxides, prepared as follows:

The cleaning atmosphere is prepared by burning anhydrous ammonia with chlorine and diluting with partially dehydrated products of the incomplete combustion of ammonia with air to produce a gaseous mixture containing (per 100 mols of the cleaning atmosphere) 5.0 mols of Hz, 1.0 mol of H20, 87.0 mols of N2, and 7.0 mols of HCl. If this cleaning atmosphere is reacted with iron oxides at 1250 F. until equilibrium is reached, the final composition of the gas would consist of (per 100 mols of original gas) 5.9 mols of Hz, 1.0 mol of H20, 87.0 mols of N2, 5.2 mols of HCl and 0.9 mol of FeClz. The concentration of FeClz in this final composition is not suificient to saturate the atmosphere with that component at the operating temperature. Therefore, this cleaning atmosphere can be used at an operating temperature of 1250" F., and, even if the reaction with iron oxides goes to equilibrium, the final gas is still not oxidizing to iron and is still not saturated with ferrous chloride.

This is a continuation-in-part of application Serial No. 595,208, filed May 22, 1945, and now abandonded.

Having described the invention, we claim:

1. A method of cleaning ferrous metal whose surface carries carbonaceous matter and iron oxides that comprises the step of exposing said metal at a temperature of 670 F.-1700 F. to an atmosphere that contains both a gas that is oxidizing to carbon and (2) a second gas that is deoxidizing to iron oxides, and so adjusting the length of time of such exposure and the concentrations of gas (1) and of gas (2) in the atmosphere during such exposure that substantially complete removal of both the carbonaceous matter and the iron oxides is accomplished simultaneously.

2. A method of cleaning ferrous metal whose surface carries carbonaceous matter and iron oxides that comprises the step of exposing said metal at a temperature of 670 F.-1700 F. to an atmosphere that contains both (1) an oxidizing gas from the class consisting of H20, 002 and mixtures thereof and (2) a deoxidizing gas from the class consisting of H2, C12, CO, HCl and mixtures thereof, and so adjusting the length of time of such exposure and the concentrations of gas (1) and of gas (2) in the atmosphere during such exposure that substantially complete removal of both the carbonaceous matter and the iron oxides is accomplished simultaneously.

3. A method as claimed in claim 1 wherein the temperature is 1000 F.-1500 F.

4. A method as claimed in claim 2 wherein the temperature is 1000 F.-1500 F.

5. A method as claimed in claim 4 wherein said atmosphere is obtained by subjecting to the operating conditions described a gaseous mixture consisting of -45 mol per cent HCl, 10-12 mol per cent CO2, not more than 2 mol per cent 02, not more than 3 mol per cent H and the remainder N2.

6. A method of cleaning ferrous metal whose surface carries carbonaceous matter and iron oxides that comprises the steps of exposing said metal at a temperature of 670 F.-1700 F. to an atmosphere that contains both (1) a gas that is oxidizing to carbon and (2) a second gas that is deoxidizing to iron oxides, and so adjusting the length of time of such exposure and the concentrations of gas (1) and of gas (2) in the atmosphere during such exposure that substantially complete removal of both the carbonaceous matter and the iron oxides is accomplished simultaneously, and then cooling said metal in an atmosphere that is inert to said metal.

7. A method of cleaning ferrous metal whose surface carries carbonaceous matter and iron oxides that comprises the steps of preheating said metal in a non-oxidizing medium and then exposing said metal at a temperature of 670 F.- 1700" F. to an atmosphere that contains both (1) a gas that is oxidizing to carbon and (2) a second gas that is deoxidizing to iron oxides, and so adjusting the length of time of such exposure and the concentrations of gas (1) and of gas (2) in the atmosphere during such exposure that substantially complete removal of both the carbonaceous matter and the iron oxides is accomplished simultaneously.

8. A method of cleaning ferrous metal whose surface carries carbonaceous matter and iron oxides that comprises the steps of exposing said metal at a temperature of 670 F.-1700 F. to an atmosphere that contains both (1) an oxidizing gas from the class consisting of H20, CO2 and mixtures thereof and (2) a deoxidizing gas from the class consisting of H2, C12, CO, HCl and mixtures thereof, and so adjusting the length of time of such exposure and the concentrations of gas 1) and of gas (2) in the atmosphere during such exposure that substantially complete removal of both the carbonaceous matter and the iron oxides is accomplished simultaneously, and then cooling said metal in a different atmosphere that is inert to said metal.

9. A method as claimed in claim 8 wherein the second atmosphere is at a higher pressure than the first atmosphere.

10. A method as claimed in claim 8 in which the initial composition of the first atmosphere includes hydrogen chloride in a, concentration low enough to form a final concentration of ferrous chloride vapor less than saturation at the operating temperature.

CARROLL CONE. JACK HUEBLER.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,199,418 Redmond May 7, 1940 2,288,980 Turin July 7, 1942 2,347,527 Vanderbilt Apr. 25, 1944 2,437,528 Hodil Mar. 9, 19 8 

1. A METHOD OF CLEANING FERROUS METAL WHOSE SURFACE CARRIES CARBONACEOUS MATTER AND IRON OXIDES THAT COMPRISES THE STEP OF EXPOSING SAID METAL AT A TEMPERATURE OF 670* F.-1700* F. TO AN ATMOSPHERE THAT CONTAINS BOTH (1) A GAS THAT IS OXIDIZING TO CARBON AND (2) A SECOND GAS THAT IS DEOXIDIZING TO IRON OXIDES, AND SO ADJUSTING THE LENGTH OF TIME OF SUCH EXPOSURE AND THE CONCEN- 